CREATE OR REPLACE MATERIALIZED VIEW

I like to add CREATE OR REPLACE MATERIALIZED VIEW with the attached
patches.

Patch 0001 adds CREATE OR REPLACE MATERIALIZED VIEW similar to CREATE OR
REPLACE VIEW. It also includes regression tests and changes to docs.

Patch 0002 deprecates CREATE MATERIALIZED VIEW IF NOT EXISTS because it
no longer seems necessary with patch 0001. Tom Lane commented[1] about
the general dislike of IF NOT EXISTS, to which I agree, but maybe this
was meant only in response to adding new commands. Anyway, my idea is
to deprecate that usage in PG18 and eventually remove it in PG19, if
there's consensus for it. We can drop that clause without violating any
standard because matviews are a Postgres extension. I'm not married to
the idea, just want to put it on the table for discussion.

Motivation
----------

At $JOB we use materialized views for caching a couple of expensive
views. But every now and then those views have to be changed, e.g., new
logic, new columns, etc. The matviews have to be dropped and re-created
to include new columns. (Just changing the underlying view logic
without adding new columns is trivial because the matviews are just thin
wrappers that just have to be refreshed.)

We also have several views that depend on those matviews. The views
must also be dropped in order to re-create the matviews. We've already
automated this with two procedures that stash and re-create dependent
view definitions.

Native support for replacing matviews would simplify our setup and it
would make CREATE MATERIALIZED VIEW more complete when compared to
CREATE VIEW.

I searched the lists for previous discussions on this topic but couldn't
find any. So, I don't know if this was ever tried, but rejected for
some reason. I've found slides[2] from 2013 (when matviews landed in
9.3) which have OR REPLACE on the roadmap:

> Materialised Views roadmap
>
> * CREATE **OR REPLACE** MATERIALIZED VIEW
> * Just an oversight that it wasn't added
> [...]

Replacing Matviews
------------------

With patch 0001, a matview can be replaced without having to drop it and
its dependent objects. In our use case it is no longer necessary to
define the actual query in a separate view. Replacing a matview works
analogous to CREATE OR REPLACE VIEW:

* the new query may change SELECT list expressions of existing columns
* new columns can be added to the end of the SELECT list
* existing columns cannot be renamed
* the data type of existing columns cannot be changed

In addition to that, CREATE OR REPLACE MATERIALIZED VIEW also replaces
access method, tablespace, and storage parameters if specified. The
clause WITH [NO] DATA works as expected: it either populates the matview
or leaves it in an unscannable state.

It is an error to specify both OR REPLACE and IF NOT EXISTS.

Example
-------

postgres=# CREATE MATERIALIZED VIEW test AS SELECT 1 AS a;
SELECT 1
postgres=# SELECT * FROM test;
a
---
1
(1 row)

postgres=# CREATE OR REPLACE MATERIALIZED VIEW test AS SELECT 2 AS a, 3 AS b;
CREATE MATERIALIZED VIEW
postgres=# SELECT * FROM test;
a | b
---+---
2 | 3
(1 row)

Implementation Details
----------------------

Patch 0001 extends create_ctas_internal in order to adapt an existing
matview to the new tuple descriptor, access method, tablespace, and
storage parameters. This logic is mostly based on DefineViewRelation.
This also reuses checkViewColumns, but adds argument is_matview in order
to tell if we want error messages for a matview (true) or view (false).
I'm not sure if that flag is the correct way to do that, or if I should
just create a separate function just for matviews with the same logic.
Do we even need to distinguish between view and matview in those error
messages?

The patch also adds tab completion in psql for CREATE OR REPLACE
MATERIALIZED VIEW.

[1] https://www.postgresql.org/message-id/226806.1693430777%40sss.pgh.pa.us
[2] https://wiki.postgresql.org/images/a/ad/Materialised_views_now_and_the_future-pgconfeu_2013.pdf#page=23

--
Erik